1. Field of the Invention
The present invention is directed to a galvanic process for galvanic or chemical treatment, in particular for the continuous application of metallic layers on a body and to a device for implementing the process.
2. Description of the Related Art
It is known in theory that the deposition rate in electrolytic transfer of material increases in proportion to increasing current densities. In practice, however, a diffusion layer forms at the cathode as current densities increase, since the transfer of matter between the anode and cathode is slower than the deposition rate of the ions in the immediate vicinity of the cathode. Thus the greater the selected current density applied, the greater the diffusion layer around the cathode and the slower and less complete the deposition rate of the ions on the cathode. Beyond a determined reaction speed, the delivery of metal ions at the phase limit between the material transfer region and charge passage region can no longer compensate for the consumption at the cathode. Therefore the current density/deposition rate curve exhibits an asymptotic limiting value which occurs, as mentioned above, due to the electrically insulating diffusion layer resulting from insufficient supply of matter. Electrolyte movement can provide a solution. As experiments have shown, the thickness of the diffusion layer decreases as the intensity of electrolyte movement increases. On the other hand, metallic deposits become rough and powdery when the selected current densities approach the theoretically possible limiting current densities. Therefore, in order to obtain satisfactory coating qualities, it is necessary to select current densities which lie far below the possible limiting current density and which, as a rule, amount to roughly only one third of the limiting current density.
In zinc deposition especially, an increased current density leads to unusable zinc deposits at the body which is to be coated owing to the present diffusion layer and the resulting poor transfer of matter. If a zinc anode is used in addition to the zinc ions in the electrolyte so as to maintain constant the percentage of metal ions for the duration of the galvanizing process, passivity effects occur at the zinc anode, since the anodic current density increases at the anode due to the dissolution process at the anode.
Arranging metal anodes on both sides of the cathode also does not lead to an improvement because this produces eccentric deposits.
DE 34 39 750 A1 discloses a process in which the electrolyte solution is moved in the direction opposite to the movement direction of the body to be coated in order to increase the deposition rate of coating materials to be applied by electrodeposition. The sum velocity resulting at the surface of the body to be coated from the two different speeds lies in the range of turbulent flow.
Although the thickness of the diffusion layer is reduced in this manner by a turbulent flow, the decomposition of the diffusion layer is insufficient. This is demonstrated, for instance, already by the fact that an upper limit of 80 to 90 A/dm.sup.2 for the current density to be applied may not be exceeded in this location. Therefore, there continues to be a diffusion layer of 10 to 15 .mu. at this location on the body to be coated.